Development of a spot test will provide a simple, economical and quick system in eukaryotes for the detection and evaluation of the mutagenic activity of chemical carcinogens and environmental agents. We have established optimal conditions for the spot test and have selected four strains from hundreds of mutants as tester strains. Conidia from 7-day-old vegetative cultures of testers are harvested and 2 x 10 to the 7th power conidia in 1 ml saline are added into 2 ml of cover medium. The mixture of conidia and cover medium is poured onto a plate containing 20 ml of Fries' minimal medium. Samples of an environmental agent are placed on the center of the plate. After 7 days' incubation at 30 degrees C, revertants from each plate are recorded. The results of our studies show that the spot test system of Neurospora can be used for screening the mutagenicity of environmental agents. We are in the process of improving our spot test system by introducing UV sensitive markers into tester strains. We are also in the process of developing a forward mutation spot test system.